Redoximorphic Paleosols in Alluvial and Lacustrine Deposits, 1.8 Ga Lochness Formation, Mount Isa, Australia: Pedogenic Processes and Implications for Paleoclimate

Abstract

ABSTRACT Paleosols in the Lochness Formation (1.8 Ga, Australia) include both rare, non-red and abundant, strongly reddened varieties that formed at subaerial exposure surfaces in both ephemeral-river and lacustrine settings. Physical processes dominated non-red paleosols, which were characterized by repeated episodes of desiccation, shrinking, and cracking alternating with wetting and introduction of sand, silt, clay, and iron oxyhydroxides into planar voids. Redoximorphic (oxidation-reduction) processes were especially intense for the red paleosols; redox depletions of Fe and Mn (hypoalbans) occur immediately adjacent to desiccation-related macropores and peds, whereas redox concentrations of Fe and Mn (quasi-coatings) occur within paleosol matrix adjacent to redox depletions. Illuviated cla and Fe-oxide coatings and hypocoatings are also common in planar macropores and on ped faces. Redoximorphic features indicate periodic water infiltration and saturation, accompanied by development of reducing conditions along planar macropores and ped surfaces in Lochness Formation paleosols. Variations in soil saturation were caused by seasonal fluctuations of lake level in lacustrine deposits, and by formation of perched saturation zones within floodplain deposits, respectively. Occurrences of red, hematitic paleosols in the Lochness Formation are compatible with previous interpretations of a weakly oxygenated 1.8 Ga paleoatmosphere. Redoximorphic features in the paleosols suggest a warm to cool temperate paleoclimate (mean annual soil temperature 5-20°C) characterized by seasonal saturation, by analogy with Quaternary redoximorphic soils. A minimal concentration of organic C (at least 1 wt %), possibly of microbial or bacterial origin, must have been present in these Proterozoic soils to allow for Fe reduction.